There has been hitherto known a technique of turning a two-phase system such as a system of an aqueous phase and an organic phase a stable state of which is a separated state in thermodynamic terms, into an emulsion that is a metastable state through emulsification.
As a typical example of emulsifying methods, there have been well known an emulsifying method using a mixer, colloid mill, homogenizer, or the like and a dispersing method using ultrasonic as described in “Emulsion Industry” published by Asakura Publishing Co., Ltd. (1971).
The typical emulsion production process is disadvantageous in that disperse-phase particles (microspheres) in a continuous phase largely vary in particle size from one another. To that end, there have been proposed a filtering method using a polycarbonate membrane, a repeatedly filtering method using a PTFE (polytetrafluoroethylene) membrane, and a method of supplying a target material to a continuous phase through a porous glass membrane having uniform pores to thereby produce a homogeneous emulsion (see Patent Document 1).
The filtering method using a polycarbonate membrane or PTFE membrane has a problem in that an emulsion cannot be produced beyond a pore size of a membrane and emulsion particles smaller than the pore size cannot be fractionated in principle. Therefore, this method is unsuitable especially for production of an emulsion having a large particle size.
Further, in the method using a porous glass membrane having uniform pores, if an average pore size of the membrane is small, a pore size distribution is small, so a homogeneous emulsion can be produced. However, if the average pore size is large, the pore size distribution becomes large, making it difficult to produce a homogeneous emulsion.
To solve the above problems, there has been proposed a process for producing a microsphere, which separates a disperse phase from a continuous phase by a partition having through-holes and applies to the disperse phase a pressure larger than that applied to the continuous phase to thereby extrude the disperse phase into the continuous phase to obtain microspheres, in which shear force is nonuniformly applied to the disperse phase extruded into the continuous phase through the through-holes to complete the microspheres (see Patent Document 2).
However, a substrate prepared by subjecting a silicon substrate to wet or dry etching based on the semiconductor microfabrication technique proposed in the specification and embodiments of the above publications is not impractical because 1) the substrate tends to get damages when in use or washing, 2) the silicon substrate costs high, and 3) through-hole width accuracy is low.
Assuming that a through-hole is formed in a silicon substrate, a substrate having the thickness of 0.1 mm to 0.3 mm is generally used. As the number of through-holes (through-hole area) is increased, mechanical strength is considerably lowered. As a result, there is a fear that the substrate is broken at the time of producing a microsphere. Thus, this processing method is not practical.
In addition, it is highly possible that the substrate is broken upon ultrasonic cleaning for reuse, for example.
The wet etching is not a precise process because high through-hole width accuracy is not secured depending on the progress of under etching below a masking material.
In contrast to wet etching, dry etching is a technique developed out of a pattern forming process of a silicon semiconductor. The application of dry etching to various electronic components with various kinds of plasma sources or a compound semiconductor has been under study. However, this process excels in microfabrication property, but an etching rate is as low as 500 to 2,000 nm/min. Thus, in the case of processing a target material with the shaping depth of, for example, 0.1 mm, a process time not Less than 50 minutes is required, so this process is not a low-cost process with high productivity.
Since the etching rate is low, reducing a substrate thickness to form a through-hole increases the possibility of breakage when in use or cleaning.
Another production process as a possible solution to the above problems is a laser processing technique. However, a general carbon dioxide laser widely used for cutting a metal or resin material or forming a through-hole has as large a laser spot size as 500 μm and thus is unsuitable for formation of small through-holes under present circumstances. In addition, there is a problem in that a process depth decreases as the spot size is more reduced with a condenser lens.
If a YAG laser having the minimum laser spot size of 30 to 50 μm is selected, the minimum possible through-hole diameter of 50 to 100 μm is realized, but laser power and directivity are low, so the upper limit of the process depth is 10 to 50 μm. Under present circumstances, the YAG laser is applied to formation of a printed wiring board or the like (see Patent Document 3).
To attain both of a small laser spot size and large process depth, laser pulse irradiation has been known. In particular, a femtosecond laser realizes the process depth of 50 μm or more with the minimum through-hole diameter of 10 to 50 μm. However, a process using the femtosecond laser is disadvantageous in that a femtosecond laser oscillator has not yet come into widespread use on an industrial scale. In addition, the oscillator is as expensive as about 100,000,000 yen per oscillator, which increases a production cost of a metal substrate having a through-hole. Moreover, as the number of through-holes of a metal substrate increases, a requisite processing period increases, and a price of a device for scanning a large area increases. Thus, its efficiency would be lowered on practical side as well.
Another production process as a possible solution to the above problems is a precision machining technique employing a precision cutting tool. However, the minimum possible bit diameter of the precision cutting tool is Φ100 μm, so it is impossible to form a through-hole with a diameter smaller than the bit diameter. In addition, machining is executed on the hole basis. Thus, it takes several hours to form several tens of thousands to several hundreds of thousand of through-holes, and its cost increases. Further, in the case of processing a large area of Φ4 inches or more (diameter of 100 mm), the precision cutting tool is worn out, resulting in a high cost.
[Patent Document 1]
    Japanese Unexamined Patent Application Publication No. 2-95433[Patent Document 2]    Japanese Patent No. 3511238[Patent Document 3]    Japanese Patent No. 2773710